Direct seismic detection of methane hydrate on the Blake Ridge

摘要

Seismic detection of methane hydrate often relies on indirect or equivocal methods. New multichannel seismic reflection data from the Blake Ridge, located approximately 450 km east of Savannah, Georgia, show three direct seismic indicators of methane hydrate: (1) a paleo bottom-simulating reflector (BSR) formed when methane gas froze into methane hydrate on the eroding eastern flank of the Blake Ridge, (2) a lens of reduced amplitudes and high P-wave velocities found between the paleo-BSR and BSR, and (3) bright spots within the hydrate stability zone that represent discrete layers of concentrated hydrate formed by upward migration of gas. Velocities within the lens (～1910 m/s) are signif- icantly higher than velocities in immediately adjacent strata (1820 and 1849 m/s). Conservative estimates show that the hydrate lens contains at least 13% bulk methane hydrate within a 2-km~3 volume, yielding 3.2 x 10~(10)kg [1.5 TCF (4.2 x 10~(10) m~3)] of methane. Low seismic amplitudes coupled with high interval velocities within the lens offer evidence for possible methane hydrate "blanking." Hydrate bright spots yield velocities as high as 2100 m/s, with bulk hydrate concentrations predicted as high as 42% in an approximately 15-m thick layer. Our results show that, under certain circumstances, hydrate in marine sediments can be directly detected in seismic reflections but that quantification of hydrate concentrations requires accurate velocity information.